There is a general need for eyepieces and viewing devices. A sector of these viewing devices finds their way into the field of head-borne (helmet-mounted) displays. When used as head borne displays an important consideration is size and weight. Reduction of the size and weight are at a premium and often other requirements are relaxed. Relaxed requirements can be optical performance at the edge of the field or distortion. However it is desirable to have low weight and small size with high optical performance and low distortion. It is the purpose of this invention to satisfy these needs by providing a compact size, low weight, excellent optical performance and distortion correction. These devices are generally used in conjunction with miniature displays. A miniature display may be a CRT or an LCD or an AMOLED or an electroluminescent device as well as other devices. All of these devices have a focal plane upon which information is displayed. The information which is displayed is generally immersed in a medium such as glass or a liquid crystal material. For example the phosphor of a CRT is generally immersed behind a glass faceplate or fiber optic faceplate, while the active area of an LCD is immersed in LC material followed by a polarizer and glass cover plate and an AMOLED phosphor is usually immersed in a potting compound and a glass cover plate.
Since the early years of head-borne displays, the importance of the spherical minor, in various forms, has been a useful tool due to the many positive optical properties associated with the concave spherical element. This concept was developed for wide field cameras used for astronomical purposes. The concept of the concentric system was used by Bernhard Schmidt (1879-1935) in “A Rapid Coma-free Mirror System” (Amateur Telescope Making, Vol. 3, Scientific American Publishing Co., New York, 1953). The concept of concentricity was explored in depth by A. Bouwers in Achievements in Optics (Elsevier Publishing Company Inc. 1950). The concept is: a concave spherical mirror with the aperture stop placed at the center of curvature, using a curved focal plane with a radius of ½ that of the mirror, placed equally between the aperture stop and the minor will yield “0” for Seidel aberration coefficients except spherical aberration and field curvature. To solve for spherical aberration, a corrector is placed at the aperture stop. To solve for field curvature, the image surface is curved. The curvature of the image surface changes the intercept points of the principal rays. The principal rays now fall at a height closer to the axis at the image plane. The lower height intercept results in a distorted image, as seen on the image plane. The above systems focus collimated light on an image plane. Geometric optics being reversible, objects at the image plane exit the aperture stop collimated. Systems used in reverse can be thought of as collimators, and due to the wide fields of view possible, these devices have found use in simulators, although with slight variations. Because these systems can have fields of view larger than 180°, the normal distortion equation using the tangent is inadequate. Since the 1960s, the term ‘mapping’ has replaced distortion. For systems covering less than 180°, mapping can still be compared to distortion in the classic sense.
During the late 1980s, a course was given by Philip Rogers (Two-Eye Visual Systems) followed by a shorter publication (Two-Eyed Optical Devices) Optics and Photonics News Vol 12, issue 7, pgs 24-27, 2001, showing many variations a spherical mirror concept to eyepieces. FIG. 1 illustrates a schematic representation of an optical apparatus for binocular viewing. This figure is from U.S. Pat. No. 4,322,135 ('135) and the '135 patent is hereby incorporated in full by reference. Several important concepts are put forth in the '135 patent including the object generated by (1) forming an image on (2) a convex fiber optic faceplate. The light passes through 5 and is reflected by 7 and 8 to a concave reflecting Surface 3 (numbers from the '135 patent. After reflection by 3 the light is transmitted through 7 and 10 and is incident on the eye, where it is perceived as an image. A relevant part of this invention is a fiber optic faceplate with a phosphor on the inner side which transforms the image plane from a flat to convex. object surface. This flat to convex transformation is required for correcting the curved field caused by the concave reflecting surface.
FIG. 2 illustrates (1) a CRT with a convex fiber optic faceplate used in conjunction with a (2) beam-splitter cube and 3 a semi-transparent concave mirror used to form a collimated image to (4) the eye. MSOD, White Plains, N.Y. sold several of these units to General Electric (Williams AFB) in 1990. The relevant part is the use of a curved surface (in this instance a fiber optic faceplate, in conjunction with a beam-splitter cube eyepiece and a concave reflector to form a head-borne display.
FIG. 3 illustrates FIG. 2 from U.S. Pat. No. 5,596,433 ('433) hereby incorporated herein in full by reference, which teaches how distortion can be corrected with the use of aspheric surfaces. In this design (r11) is used to correct for distortion caused primarily by (R). The relevant part is a distortion corrected beam-splitter cube eyepiece through the use of aspheric optical surfaces.
FIG. 4 is from U.S. Pat. No. 5,596,451 ('451), hereby incorporated herein in full by reference, illustrates a CRT in combination with a polarizing beam splitter cube, a ¼ wave plate, and curved reflectors that are used to increase the transmission of the devices shown in FIG. 2. The relevant part is the manipulation of polarized light by using ¼ wave plate(s) to rotate the plane of polarization thereby using a trans-reflecting polarizing surface within the cube to improve system transmission.
FIG. 5 is from U.S. Pat. No. 3,443,858 ('858), hereby incorporated herein in full by reference, and illustrates a method of making a compact collimating optical system by selective use of polarized light. The relevant part is the compacting of a device, such as a spherical mirror/beam splitter, to form a smaller length.
The attributes of the prior art can be arranged to form variations. The aspheric surfaces of FIG. 3 can be combined with FIG. 4 to produce a reduced distortion version of FIG. 4. A convex fiber optic screen can be used with FIG. 3 to correct for field curvature thus requiring flatter aspheric surfaces. The CRT phosphor shown in FIG. 2 can be replaced with another phosphor such as an OLED phosphor or an electroluminescent phosphor. However, no combination of the examples allows for the correction of distortion on a flat plane using spherical surfaces.
However, it is possible to combine the above technologies to produce an eyepiece, such as a cube eyepiece, as shown in FIG. 2, to form a distortion free image if a fiber optic faceplate is used. The concept of using a fiber optic plate which is plano on the phosphor side and in contact with an OLED phosphor and a convex opposite side combined with a cube eyepiece was demonstrated to NVIS Inc. Reston, Va. prior to this application with regard to using this concept in an Army SBIR proposal A04-222, now Contract W91 RB08C 0151. FIG. 6 illustrates the optical arrangement of this device. FIG. 7 illustrates the astigmatism and distortion for a centered pupil. FIG. 8 illustrates the quotation for the convex fiber optic faceplate with material specified by eMagin. However this work was done by the assignee in 2006. This work was produced by the assignee as a partner with NVIS in an SBIR proposal and is included herein as prior art.